The invention relates to a method of manufacturing a semiconductor device suitable for obtaining multilayer wiring.
In a VLSI process, borophosphosilicate glass (hereafter abbreviated as a BPSG film) is utilized for an inter layer dielectric film of a multi-level metallization. When using the BPSG film as the layer insulation film, the BPSG film is normally passed through two high temperature processes (flow process and reflow process) to level the unevenness of a lower wiring or to slope a contact hole.
The manufacturing method employed conventionally of the MOS type semiconductor device using the flow process and the reflow process of the BPSF film will be explained by referring to the process charts a through c of FIG. 3. In this manufacturing method, on one main side of a silicon substrate 21 are formed a selective oxidized film 22 (LOCOS0, a gate oxidized film 23, a polycrystal silicon gate 24, and a diffusion layer 25, and further a silicon nitride film 26 is heaped all over the surface of the substrate. Then, the BPSG film 27 of 3 weight percentage of boron concentration and 4 weight percentage of phosphorus concentration is heaped on the substrate, and then, in order to flatten the surface of the BPSG film 27, the substrate is heat treated (flow process) in oxygen at 900.degree. C. for 60 Minutes. Following the heat treatment, a contact hole 28 is formed in the BPSG film 27 and the silicon nitride film 26 (FIG. 3, a). Then, the substrate is heat treated (reflow process) in nitrogen at 900.degree. C. for 30 minutes. By this heat treatment, a shape 28a of the step saction of the contact hole 28, which was steep after the above heat treatment, was made round (FIG. 3, b).
Finally the MOS type transistor is completed by forming an aluminum interconnection 30 as the upper wiring (FIG. 3, c).
However, in this case, an overhang is formed on the side wall section of the contact hole 28 during the heat treatment of the reflow process, by a fine contact hole 28 formed in the BPSG film 27. The step coverage of the aluminum interconnection 30 in the contact hole is caused to become worse by this overhang, leading to possible trouble of breakage of the conductor or of electromigration.
The overhang of the contact hole can be alleviated by reducing the boron concentration or phosphorus concentration of the BPSG film 27 and controlling the extent of the reflow process, but in this case, the BPSG film 27 cannot be flattened sufficiently during the flow process. On the other hand, if the heat treatment temperature of the reflow process lowered or if the time for heat treatment is shortened to control the overhand of the BPSG film 27, it is not possible to form a roundness or a slope at the peripheral edge 28a of the contact hole 28, and the step coverage of the aluminum interconnection 30 cannot be improved.